The one thing I would like to un-invent would be weapons of mass destruction. There is no need for them and only cause fear and devastation which the world have never seen before.
If I had a second un-invention it would be robocalls. There are so many of them and they have all new spams that call for car warranties, past due tax bills, unwanted tech support, etc. Luckily my phone has a feature to block unknown callers that don’t appear in my contacts. So perhaps this was already reversed engineered!